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Instead of facing a murder charge, she took a plea agreement allowing the case to move to the punishment phase Tuesday.

Matthews could face up to 99 years to life in prison, prosecutor Kimberly D'Avignon said.

In testimony Tuesday, prosecution witnesses painted Matthews as an absent mother at best. Her mother, Suzette Edwards, has custody of Matthews' two older daughters, and a family friend has adopted two younger daughters who were living with Matthews at the time of their brother's death.

Mike Weber, an investigator with the Tarrant County district attorney's office, testified that a runaway, then 16, encountered Matthews' children in summer 2009. Brittney Smith, now 18, told the jury that she saw Matthews' two younger daughters walking home near Collins Street in Arlington and had to pull one out of the street because she was almost run over.

Smith said she walked the girls home, and one of them gave her the door key. The mother was absent, Smith said.

"I heard them say 'Hi Coco' and I looked around but didn't see a dog," Smith said. "Then I saw the baby run across the floor, some trash here and there, and no furniture."

Smith said the baby's diaper was heavy, as though it had been soiled several times, and it was tearing in the back. There were pans with hush puppies and fries in the kitchen, and a black bowl on the floor containing food she assumed was for the toddler, Smith said.

Smith told the older daughter that she would return every five to 10 minutes to check on them and kept watch close by, afraid to call police because she was a runaway.

Smith said she returned "three or four times" before she saw Matthews on the floor changing the toddler's diaper.

"I gave her the key and she didn't ask me anything," Smith said. "I tried to explain to her what happened, but it was like it was nothing."

About a week later, Child Protective Services employees took Smith into custody, and weeks later Weber questioned her about Matthews.

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The jury also heard testimony from Arlington Detective Ben Lopez and a taped interview he conducted with Matthews shortly after the death.

Matthews lied to several officials before finally admitting that she had left her son in a hot vehicle.

Matthews said that she checked on the toddler five or six times before leaving work and that he only looked flushed when she left to pick up her other children. The next morning, Matthews called the detective and corrected her earlier statement.

"Naturally, the jury is not going to be as sympathetic to someone who has lied," said Robin McCarty, defense attorney. "But it was only a short time later, that next morning, when she corrected her statement. She was obviously very scared."